THE ROLE OF NITRIC-OXIDE IN THE TRANSFORMATION OF VISUAL INFORMATION WITHIN THE DORSAL LATERAL GENICULATE-NUCLEUS OF THE CAT

We have shown that application of an inhibitor of the enzyme nitric ox ide synthase (NOS) effectively suppresses the visual responses of rela y cells in the dorsal lateral geniculate nucleus (dLGN) of the anaesth etized paralysed cat. Such suppression seems to result from a specific reduction in transmission via N-methyl-D-aspartic acid (NMDA) recepto rs, since iontophoretic application of the inhibitor of NOS selectivel y and in a dose-dependent manner decreased the responses to exogenousl y applied NMDA. Responses to other exogenously applied amino acid agon ists, such as quisqualate (Quis), kainate (Kain) and a-amino-3-hydroxy -5-5-methyl-4-isoxazole-propionic acid (AMPA) were largely unaffected. Furthermore, the excitatory action of acetylcholine (ACh), normally c o-localized with NOS in axonal terminals within the dLGN arising from the brainstem, was also unaffected. Unlike some other actions of nitri c oxide (NO), this role seems not to involve an increase in production of cyclic guanosine-3',5'-mono-phosphate (cGMP), since application of the membrane permeable cGMP analogue 8-bromo-cGMP did not alter the s uppressive effect of NOS inhibitors on either visual or NMDA evoked re sponses. We conclude that the normal function of NO at this level of t he visual system is permissive, allowing full expression of NMDA media ted visually elicited information.